Turnips have become a popular forage crop in New Zealand and Australia for supplementing dairy cows grazing pasture during the summer and autumn. However, it is unclear what combination of turnips and summer pasture best meets the dietary requirements of lactating dairy cows. New information was therefore gathered on the digestibility of turnips fed in conjunction with summer pasture and this was used in the ration balancing model, SPARTAN, to determine the optimum combination of turnips and summer pasture for a `large` dairy cow (500 kg, 17 l/d) during mid-to late-lactation. Twelve wethers (n = 3/group) were used to determine the in vivo digestibility of four diets with a summer pasture (P):turnip (T) dry matter (DM) ratio of: 100P:0T, 75P:25T, 50P:50T and 0P:100T. The DM digestibilities of the diets, fed at 1.0 kg DM/d, were 65, 71, 81 and 89%, respectively. As the portion of turnips in the diet increased so too did the dry matter intake (0.64, 0.78, 0.77 and 0.93 kg DM/d, respectively). Relative to pasture, turnip was low in fibre and crude protein but high in available carbohydrate. The optimum portion of turnips, on a DM basis, when fed in conjunction with summer pasture was 21% of the dietary intake. For the average New Zealand dairy cow, this equates to 3-5 kg turnip DM/d. A 100% turnip diet would not meet cow requirements because of the low fibre and protein content of turnips, but a 100% pasture diet could, providing it was reasonable of quality (ca. 65% DMD and at least 17% crude protein) and available in sufficient quantity not to restrict cow intake.
Proceedings of the New Zealand Society of Animal Production, Volume 57, , 161-164, 1997
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